Before the United States dropped atomic bombs on Hiroshima and Nagasaki in August 1945, very few civilian or military leaders knew of the weapon's existence, including most of the U.S. Congress. As vice president, Harry S. Truman was likewise unaware of the bomb's existence, and was informed only after he assumed the presidency on April 12, 1945 -- a scant four months before he authorized its use against Japan.
After being informed of the existence of the atomic bomb, President Truman convened his top military advisers and scientists -- the "Interim Committee" -- to discuss the possible ramifications of using the atomic bomb. The committee was unanimous in recommending to use the bomb on Japan and nearly unanimous -- one member dissented -- in recommending to do so without warning the Japanese about its potential destructive power.
As commander in chief, Truman had the constitutional authority to order use of the weapon without congressional approval, which he did. An American bomber dropped the first atomic bomb on Hiroshima on August 6, 1945. The same day, Truman publicly announced the atomic bomb's existence and its use against Japan. At the end of his address, Truman said he would recommend that Congress form a commission to control U.S. use of atomic power. After a second bomb was dropped on the city of Nagasaki three days later, the Japanese sent word that they would surrender, forestalling a third attack.
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